1. Field of the Invention
The invention relates generally to global positioning system (GPS) receivers and more particularly to a GPS receiver apparatus having a low power consumption and fast time to first fix by receiving GPS information from a message system.
2. Description of the Prior Art
In low-power portable GPS receivers, there is a strong need to provide GPS-derived location and time reports using the least amount of power and in the shortest time. In several applications for GPS receivers, such as tracking railcars, ship containers, or truck trailers, it is desirable to achieve a very low average power by finding a location fix as rapidly as possible and then turning off or to a low power standby mode until it is time to find another location fix minutes, hours, or days later.
Currently, upon a cold start, when a GPS receiver has no knowledge of location, time, almanac or ephemeris, the receiver has to search a GPS signal for all possible GPS satellites to find the carrier frequencies and code phases from the GPS satellites that are visible. Once one or more visible GPS satellites are acquired and tracked, the GPS receiver reads GPS time and ephemeris information from the GPS signal before it can derive a GPS-based geographical location and local time. In existing GPS receivers, these steps take upwards of two minutes and sometimes more to complete and require the GPS receiver to operate at full power. Therefore, a substantial amount of energy is required to produce the first location fix from a cold start. In the case of a warm start when the GPS receiver has an approximate knowledge of location, time, and almanac, the time to acquire and track the visible GPS satellites is shortened. Even so, the GPS receiver must monitor the GPS signal for eighteen to forty-eight seconds or more to read the GPS time and the ephemeris information which still requires a substantial amount of energy.
Several systems exist or have been proposed having smart search algorithms to reduce the time in searching for the visible GPS satellites from among all the possible GPS satellites. Lau in U.S. Pat. No. 5,418,538 goes one step further by disclosing a system that uses a radio broadcast signal having differential GPS corrections to determine which GPS satellites are visible. However, none of the existing systems meet the need of applications where fixes are only required every few minutes, hours, or days but very low average power is required.